370 VEGETABLE PHYSIOLOGY 



effect of any modification of the conditions which have 

 induced tone. We have seen, for instance, that a particular 

 degree or range of illumination sets up in a plant the con- 

 dition of phototonus, which is one constituent of the healthy 

 tone of the organism. Any modification of that illumination 

 is followed by certain effects, the extremes of which we 

 have already discussed. This alteration of the optimum 

 illumination becomes at once a stimulating action, and 

 we can speak of a stimulating influence of light, which is 

 really any change in what we have called its tonic action. 

 It can be in the direction of increase or decrease of the 

 latter, but as .it induces changes it must be regarded as 

 stimulating. 



What is true of light is also true of the other factors 

 which combine to produce the healthy tone of the plant. 

 Changes of temperature bring the organism nearer to or 

 further from that optimum point at which it is in the most 

 complete state of thermotonus and are responded to in 

 various ways accordingly. Any alteration in the fluid 

 contents of a cell brings about a change in what we may 

 call the tonic tension of that cell, in which condition the 

 permeability of the protoplasm exists at its best, and again 

 an appropriate response is made. 



In considering broadly the result of stimulation we 

 must notice at the outset that it provokes a purposeful 

 response. The living substance appears to have a definite 

 aim ; it may be to remove the stimulating cause if the 

 latter affects it prejudicially ; it may be to readjust its 

 manifold forces to the new conditions to which the environ- 

 ment is suddenly or gradually subjecting it. 



The means which the plant avails itself of are seldom 

 abrupt and violent, like the manifestation of muscular con- 

 tractility, but more frequently take the form of the modifi- 

 cation of some rhythm which is characteristic of its 

 behaviour. A few cases of sudden and sharp change are 

 met with, as when the leaf of Mimosa droops on being 

 touched, or when that of Dionaa rapidly closes over its 



